This invention relates generally to a method for controlling emissions and, more specifically, to a method for controlling emissions by controlling cold start intake manifold pressure in a parallel hybrid motor vehicle.
When an internal combustion engine (ICE) in a motor vehicle is initially started (especially in a cold climate), the interior surfaces of the engine are cold. In addition, because the engine is initially turning at a very low RPM, the intake manifold absolute pressure (MAP) is near atmospheric pressure. Because liquid fuel does not combust as easily or cleanly as gaseous fuel, it is desirable that the fuel sprayed into and mixed with the air jet traveling into the combustion cylinders of the engine be vaporized in order to reduce emissions from the ICE. Unfortunately, both the relatively high MAP and the cold condition of the engine make it difficult to vaporize the fuel injected into the combustion cylinders. Therefore, in order to produce the desired amount of power at start-up and during high-torque initial accelerations shortly after start-up when the engine is still cold, additional (i.e., excess) amounts of fuel must be pumped into the intake manifold to obtain a sufficient amount of vaporized fuel. All of the additional fuel is not completely vaporized and the incompletely vaporized fuel is not completely combusted. The consequence of the poor fuel vaporization at startup and during initial high-torque accelerations is increased emissions. The excess fuel that is not completely combusted at start-up and the period shortly after start-up creates an exhaust mixture that is too fuel-rich to be stoichiometric at the catalytic converter, thus leading to increased hydrocarbon and carbon monoxide emissions.
Under most operating conditions, once the intake valves have heated up adequately (usually within about 60 seconds after engine start-up), the excess fuel is no longer necessary, as the intake valves are hot enough to properly vaporize the injected fuel. At this time, the engine RPM is also high enough to provide a low MAP, assisting with the fuel vaporization. Even in high torque-demand situations, such as during acceleration, which causes the MAP to increase, the hot intake valves are able to vaporize the fuel so that it combusts thoroughly.
High emissions, however, can also result from rapid changes in MAP even with the engine heated, as well as from the high MAP at startup. When there is a rapid drop in torque demand, such as at the end of a rapid acceleration, the throttle closes and the MAP will quickly drop from the high MAP consistent with the rapid acceleration to a low MAP consistent with the lower torque demand. Any liquid fuel left in the intake manifold after the throttle closes rapidly flashes to a gaseous state because of the low MAP and the hot engine components. There is usually too little of this gaseous fuel to fully combust; the fuel-air mixture is too lean (has too much air present) to properly and completely ignite in the cylinder combustion chamber. The unburned fuel-air mixture is exhausted and passes to the catalytic converter. This unburned fuel-air mixture again leads to increases in the hydrocarbon and carbon monoxide emissions.
Air injection reaction (AIR) systems have been employed as one means to reduce the emissions resulting from start-up and from the driving immediately thereafter by pumping air into the exhaust manifold. The injected air helps provide the catalytic converter with a stoichiometric mixture of unburned fuel and air. Additionally, advanced engine controls and advanced fuel-swirling devices have been used to provide a more easily ignited air/fuel mixture for injection into the cylinders. Problems exist, however, with both of these approaches. The AIR system is only used for about 20 seconds at the initial start-up of the motor vehicle and has no function thereafter in the operation of the motor vehicle. The AIR system adds weight and complexity (and thus cost) to the motor vehicle and yet is only functionally necessary for a short period of time at cold start-up. The advanced engine controls and fuel-swirling techniques also add complexity and cost to the motor vehicle. Both of these methods help primarily with reducing start-up and initial emissions and are largely unable to reduce emissions in other driving situations such as conditions resulting in rapid changes in MAP.
Presently available methods for controlling emissions during cold start-up and in situations of rapid changes in MAP are costly and/or ineffective. Accordingly, a need exists for an improved emissions control method that will provide cold start intake manifold pressure control and that can provide a reliable method of limiting both the range of the intake manifold absolute pressure and the rate of change of intake manifold absolute pressure in the interests of lowered emissions throughout the operation of the motor vehicle.
In accordance with one embodiment of the invention, a method is provided for controlling emissions in a parallel hybrid motor vehicle that includes an electric propulsion system in parallel with a combustion propulsion system. Manifold absolute pressure (MAP) is monitored in the intake manifold of the combustion propulsion system. The electric propulsion system is engaged to reduce the MAP measured in the intake manifold to a predetermined pressure, and then fueling and combustion of the combustion propulsion system are initiated only after the MAP is reduced to a pressure less than the predetermined pressure.